Surface Morphology of Al_xGa_1-xN Films Grown by MOCVD

The surface morphology of Al_xGa_1-xN alloys has been studied as a function of aluminum composition using AFM. The specimens were grown by standard MOCVD techniques and exhibit superior surface morphology, crystalline structure and electronic properties. Atomic force microscopy was used to study the surface morphology of a series of Al_xGa_1-xN alloys, where the nominal aluminum content ranges from 8% to 37%. We observe four types of surface features: atomic steps, terraces, pinholes, and clusters of pinholes. The pinhole density remains the same in all samples at -10⁹ cm^-2. Atomic steps of one or two monolayers are observed on terraces. Clustering of pinholes is found at high aluminum contents. We attribute these morphological features to dislocations originating at the GaN buffer layer. The pinholes at the surface can be explained as extended coreless dislocations that form crater-like features near the free surface of the crystal. The atomic steps on the surface are due to established step-growth kinetics mediated by dislocations. Mixed-type dislocations lead the surface crystal growth while coreless screw dislocations tend to retard the surface crystal growth. We establish a clear correlation between dislocations and surface morphology for a wide range of aluminum composition in Al_xGa_1-xN alloys.